The present invention involves methods and compositions for treating subterranean formations, and more specifically, relates to improved methods and compositions for degrading filter cake deposited in a subterranean formation by effecting a uniform distribution of a filter cake breaker.
Filter cake, the residue deposited on a permeable medium when servicing fluids contact the medium under pressure, is formed in a variety of subterranean operations such as drilling, fracturing, and gravel packing. A filter cake is often desirable, at least temporarily, in subterranean operations as it may act to stem the flow of a servicing fluid from its desired location, to the surrounding subterranean formation. For instance, where the servicing fluid is a drilling fluid, a filter cake formed on the wall of the well bore may act to keep the drilling fluid in its desired location, in the annulus between the well bore and the drill pipe, rather than allowing the drilling fluid to leach off into the surrounding formation. Losses of drilling, fracturing, gravel transport, and other servicing fluids into the formation represents an increased expense and, loss of too much fluid may damage producing zones in the formation. Moreover, in soft or poorly consolidated sandstone formations, the presence of a filter cake may add strength and stability to the formation surfaces on which the filter cake forms. Without the filter cake in place and differential pressure applied to the filter cake, poorly consolidated or unconsolidated formations may suffer from hole collapse. Keeping the drilled bore hole open is essential for the installation of completion equipment.
Sand control operations, such as gravel packing, are performed after a well bore is drilled. One common type of gravel packing operation, referred to as “open hole” gravel packing, involves placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel of a specific size designed to prevent the passage of formation sand. The gravel pack screen is generally a filter assembly used to support and retain the gravel placed during gravel pack operation. A wide range of screen sizes and configurations are available to suit the characteristics of the gravel pack gravel used. Similarly, a wide range of sizes of gravel is available to suit the characteristics of the unconsolidated or poorly consolidated particulates in the subterranean formation. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow.
When installing the gravel pack, the gravel is carried to the formation in the form of a slurry by mixing the gravel with a transport fluid. Gravel packs act, inter alia, to stabilize the formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the production tubing. Often, gravel packs are placed along a well bore having a filter cake on its walls.
While filter cakes may be beneficial during treatment operations, it is generally necessary to remove filter cakes from producing zones prior to the well being placed onto production. The removal of filter cakes from producing formations has previously been accomplished by including an acid-soluble particulate solid bridging agent in the material that forms the filter cake. Such an acid-degradable filter cake could then be treated by placing an acid solution in contact with the filter cake and allowing that solution to remain in contact for a period of time sufficient to dissolve the bridging particles and, thus, degrade the filter cake. These post-completion acid treatments are generally placed through coil tubing and are expensive and time consuming to apply.
Post completion, filter cake cleanup treatments have only been partially successful because of the gravel pack and sand screen act as a barrier to contacting the filter cake with the cleanup solution. In addition, the highly reactive nature of the acid causes instantaneous degradation of the filter cake in those areas where the contact occurs which leads to high localized fluid loss, making the remainder of the treatment difficult to apply uniformly.
One conventional method that attempts to overcome that problem involves placing a slow reacting or delayed material suitable for degrading a filter cake, e.g., an oxidizer, ester, enzyme, or the like (hereinafter a “breaker”) in the gravel pack carrier-fluid in hopes that the breaker will be uniformly distributed throughout the gravel pack. However, because the breaker is dissolved in the gravel pack carrier fluid, and not all of the gravel pack carrier fluid remains in the subterranean formation, much of the breaker that is pumped gets circulated out of the well bore and does not interact with the filter cake as desired. Improved methods deliver the breaker as a particle that deposits with the gravel pack since all of the particles remain in the screen/well bore annulus.
It is also known in the art that a breaker may be encapsulated to delay its release. Encapsulating a breaker would allow it to be delivered as a particle that could be deposited with the gravel pack. By so delaying the release of the breaker the user is able to place the breaker in a subterranean formation and to have some time before the encapsulated material begins to substantially act as a breaker. However, such encapsulated breakers known in the art generally exhibit low densities, less than about 1.8. When the encapsulated breaker particles are sent into a subterranean formation as part of a gravel packing operation, they often do not deposit uniformly throughout the pack. This is because the specific gravity of the gravel is generally at least about 2.5. This is especially problematic in horizontal wells where there would be a risk of the gravel depositing along the bottom of the well bore and the lighter encapsulated particles being carried further along the well bore.